Non-invasive system and method for tracking bones

ABSTRACT

Systems and methods for determining position and orientation of a bone of an anatomical feature are described. These include the use of a wearable holder configured to be mounted about an outer-skin surface of the anatomical feature, such that the anatomical feature and the bone are positioned in fixed relation with respect to the wearable holder when the wearable holder is mounted about the anatomical feature. Reference marker arrays are fixedly mounted to the wearable holder, each being positioned on the wearable holder to identify a landmark of the bone within the wearable holder when the wearable holder is mounted to the anatomical feature. The position and orientation of the reference markers are trackable to determine position and orientation of the wearable holder in a reference coordinate system, thereby enabling position and orientation of the landmarks on the bone to be determined.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority on U.S. Patent Application No.62/482,720 filed Apr. 7, 2017, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to the field of computer-assisted medicalprocedures and, more specifically, to bone tracking and positioning incomputer-assisted surgery (CAS) systems.

BACKGROUND

Computer-assisted surgery (CAS) makes use of markers fixed to thepatient to track bones before and during surgery. Conventional trackablemarkers often include surgical pins which are inserted into the bones tobe tracked. These pins, inserted into the bones before or during thesurgery, may be of different diameter sizes and are relatively invasive.The insertion of such pins adds an extra step to the surgery and do notcontribute to treatment, their only purpose being to assist surgicaltracking of the bone. The installation of such pins can also betime-consuming.

Furthermore, the length of the pins is sometimes obtrusive to thesurgeon who may therefore cut them to a length better adapted to her orhis movement during the surgery. The need to sometimes cut the pins inthis regard is also perceived as an additional step not particularlyliked by surgeons, and the cut ends of pins may be sharp and hazardousto the personnel working around the surgery table.

An alternate option for tracking bones is to position optical trackersin relation to the bone to be tracked, for instance on soft tissue or onthe pins themselves. However, in such a scenario, care must be taken toavoid movement of the optical trackers on the soft tissue, which mayhamper the surgeon's maneuvers. Optical tracking is not always suitablebecause personnel or another object may block the optical signal, andtherefore obstruct the line-of-sight between the sensor and the opticaltracker.

SUMMARY

In accordance with one aspect of the present disclosure, there isprovided a system for determining a position and an orientation of abone of an anatomical feature, the bone underlying an outer-skin surfaceof the anatomical feature, the system comprising: a wearable holderconfigured to be mounted about the outer-skin surface of the anatomicalfeature, the anatomical feature and the bone being positioned in fixedrelation with respect to the wearable holder when the wearable holder ismounted about the anatomical feature; at least two reference markerarrays fixedly mounted to the wearable holder, each said referencemarker array being positioned on the wearable holder to identify alandmark of the bone underlying the wearable holder when the wearableholder is mounted about the anatomical feature, each said referencemarker array including a plurality of reference markers; a fixedreference defining a reference coordinate system; a position sensingdevice operable to register position and orientation readings of thereference markers in the reference coordinate system; and a processingunit operable to receive the position and orientation readings and tomeasure time intervals, the processing unit being operable to determinethe position and the orientation of the holder in the referencecoordinate system for a given time interval.

In the system as defined above, the wearable holder may include a bootthat is removably mountable about a foot and a lower leg.

In the system as defined above, the arrays may be positioned on the bootto overlie the landmarks of the foot and lower leg, the landmarksincluding medial and lateral malleoli of the ankle when the boot ismounted about the foot and the lower leg.

In the system as defined above, the reference marker arrays may befixedly mounted to an immobile portion of the boot.

In the system as defined above, the reference markers may include activereference markers, each reference marker being operable to emit anelectromagnetic wave, receive an echo of the wave off of a surface ofthe landmark, and record a time measurement for the echo.

In the system as defined above, the fixed reference may include aplurality of trackers fixedly mounted to a surgery table, each trackerbeing spaced apart from one another to define the reference coordinatesystem.

In the system as defined above, at least one of the reference markersmay include an ultrasound device.

In the system as defined above, the fixed reference may include at leastone ultrasound tracker.

In the system as defined above, the holder may include a belt beingremovably mountable about a leg.

In accordance with another aspect of the present disclosure, there isalso provided a method for determining a position and an orientation ofa bone of an anatomical feature, the bone underlying an outer-skinsurface of the anatomical feature, the method comprising: removablyattaching in a non-invasive manner a holder about an anatomical featurehaving the bone to fix a position of the anatomical feature and the bonewith respect to the holder; identifying at least two landmarks of thebone, the landmarks being fixed in position with respect to the holder;registering position and orientation readings of reference markersfixedly mounted to the holder in a reference coordinate system; anddetermining the position and the orientation of the holder using theposition and orientation readings of the reference markers to therebydetermine the position and orientation of the landmarks of the bone inthe reference coordinate system.

In the method as defined above, identifying the at least two landmarksmay include identifying the at least two landmarks without imagery ofthe bone.

In the method as defined above, identifying the at least two landmarksmay include measuring a distance between each reference marker and thecorresponding landmark on the bone.

The method as defined above may further include tracking the landmarksof the bone by repeating said registering and said determining atdifferent time intervals.

In the method as defined above, registering the position and orientationreadings may include triangulating the position and orientation readingsof the reference markers with at least two reference trackers fixed inposition within the reference coordinate system.

In the method as defined above, identifying the at least two landmarksmay include pre-operatively imaging the at least two landmarks.

In accordance with another aspect of the present disclosure, there isalso provided a wearable holder that is trackable by a computer-assistedsurgery system, the wearable holder comprising: a body removablymountable about an outer-skin surface of an anatomical feature having anbone, the anatomical feature and the bone being positioned in fixedrelation with respect to the body when the body is mounted to theanatomical feature; and at least two reference marker arrays fixedlymounted to the body, each reference marker array being positioned on thebody and operable to identify a landmark of the bone when the body ismounted about the anatomical feature, each of the reference markerarrays having a plurality of reference markers, a position and anorientation of the reference markers being trackable in a referencecoordinate system to determine the position and the orientation of thebody in the reference coordinate system.

In accordance with another aspect of the present disclosure, there isfurther provided a system for determining a position and an orientationof bone of an anatomical feature, the bone underlying an outer-skinsurface of the anatomical feature, the system comprising: a wearableholder removably mountable about the outer-skin surface of theanatomical feature, the anatomical feature and the bone being positionedin fixed relation with respect to the holder when the wearable holder ismounted about the anatomical feature; at least one holder imaging devicefixedly mounted to the wearable holder to obtain an image of a landmarkof the bone at a given time interval; a bone imaging device operable toobtain a pre-operative image of the bone in its entirety, the positionand the orientation of the bone in the pre-operative image being knownin a reference coordinate system; and a processing unit operable tocompare the image of the landmark at said time interval to the samelandmark in the pre-operative image, the position and the orientation ofany portion of the bone in the reference coordinate system at said timeinterval being determined by matching the image of the landmark to thesame landmark in the pre-operative image.

In the system as defined above, the bone imaging device may be operableto construct the pre-operative image of the bone from at least twoimages taken in non-coplanar planes.

In the system as defined above, the image of the landmark may beobtainable by the at least one holder imaging device is a contour of thebone underlying the wearable holder.

The system as defined above may further include a plurality of referencemarkers fixedly mounted to the holder, the position and the orientationof the reference markers being trackable in the reference coordinatesystem to determine the position and the orientation of the holder inthe reference coordinate system.

In the system as defined above, the wearable holder may include a firstprobe mountable about the outer-skin surface of an upper portion of theleg having an underlying femur, and a second probe mountable about theouter-skin surface of a lower portion of the leg having the underlyingtibia.

In the system as defined above, the at least one holder imaging devicemay include an ultrasound imaging device.

In accordance with yet another aspect of the present disclosure, thereis further provided a method for determining a position and anorientation of a bone of an anatomical feature, the method comprising:obtaining a pre-operative image of the bone in its entirety, theposition and the orientation of the bone in the pre-operative imagebeing known in a reference coordinate system; removably attaching in anon-invasive manner a holder to the anatomical feature to fix a positionof the anatomical feature and the bone with respect to the holder;obtaining an image of a landmark of the bone at a given time intervalwith the holder, the landmark being fixed in position with respect tothe holder; and determining the position and the orientation of anyportion of the bone in the reference coordinate system at said timeinterval by matching the image of the landmark to the same landmark inthe pre-operative image.

In the method as defined above, obtaining the pre-operative image mayinclude constructing the pre-operative image from at least two imagestaken in non-coplanar planes.

In the method as defined above, obtaining an image of the landmark mayinclude obtaining a contour of the bone underlying the holder.

The method as defined above may further include registering position andorientation readings of reference markers fixedly mounted to the holderin the reference coordinate system, and determining the position and theorientation of the holder using the position and orientation readings ofthe reference markers to thereby determine the position and orientationof the landmarks of the bone in the reference coordinate system.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a perspective view of a holder being trackable in acomputer-assisted surgery system, according to an embodiment of thepresent disclosure;

FIG. 2 is a diagram of a system for determining a position and anorientation of bone, which underlies an outer-skin surface, in space;

FIG. 3 is a block diagram of a processing unit of the system of FIG. 2;and

FIG. 4 is a diagram of another system for determining a position and anorientation of the bone in space.

DETAILED DESCRIPTION

FIG. 1 illustrates a wearable holder 10 (or simply “holder” 10 asreferred to herein) that is removably mounted about the outer-skinsurface 11 (a.k.a., exposed skin, epidermis, external soft tissue, etc.)of an anatomical feature, such as but not limited to a lower leg portion13 of a patient. The wearable holder 10 and the system using it, as willbe described herein, may therefore be used to determine the positionand/or orientation of body parts other than the lower leg portion 13 ofa patient, such as anatomical features of arms (elbows, wrists, hands,etc.), other knee joints (e.g. knees, hips, etc.), and the like. Thelower leg portion 13 includes a bone 14, which is a tibia 15 in thedepicted embodiment. The bone is largely subcutaneous, in that amajority thereof is disposed beneath, and thus substantially underlies,the outer-skin surface 11 of the anatomical feature in question. Incertain embodiments, the bone may thus be said to be substantiallyunexposed. However, it is to be understood that one or more portions ofthe bone 14 may be exposed during surgery, for example as a result ofone or more incision(s) made as part of the surgical technique beingemployed. Accordingly, while portions of the bone 14 may be exposedduring surgery with the anatomical feature within the holder 10, thebone will otherwise remain substantially subcutaneous. While the bonemay be described herein as “underlying” the outer-skin surface, it is tobe understood that this does not exclude the possibility that certainportions of the bone may at least partially exposed during surgery (e.g.by incisions, etc.) nor does this require or imply that the entirety ofthe bone must necessarily be unexposed and subcutaneous at all times.

The holder 10 is configured to be secured to the anatomical featureagainst which it is mounted in such a way that there is negligible or nomovement between the holder 10 and the anatomical feature. The positionand the orientation of the holder 10 are also trackable through space,whereby a tracking of the anatomical feature can be derived from atracking of the holder 10. The holder 10 is therefore a non-invasivetool to be used to track the position and the orientation, and thus themovement, of the bone 14 through space. It is therefore appreciated thatthe holder 10 can take different forms to accomplish such functionality.

The holder 10 is in the form of a boot 10A in the depicted embodiment.In an alternate embodiment, the holder 10 is in the form a belt that ismounted to an anatomical feature (e.g. an arm and the underlying humerusbone, a leg and the underlying femur, etc.) of the patient to be infixed relative relationship with the bone 14. In another alternateembodiment, the holder 10 is in the form a tight-fitting sleeve that ismounted to an anatomical feature of the patient to be in fixed relativerelationship with the bone 14. Similarly, the holder 10 is mountableabout other limbs, appendages, or other anatomical features of thepatient having a bone 14 to be tracked.

When the holder 10 is mounted to the lower leg portion 13, theanatomical feature of the patient (the lower leg portion 13 in theillustrated embodiment) and the bone 14 (the tibia 15 in the illustratedembodiment) are in fixed relation with respect to the holder 10. Theholder 10 is mounted about the lower leg portion 13 to substantiallyprevent the lower leg portion 13 and the foot from moving relative tothe holder 10. This allows the position and the orientation of the lowerleg portion 13 with respect to the holder 10 to remain substantiallyconstant, and helps to track the movement of the bone 14 through space,as explained in greater detail below.

Two or more reference marker arrays 20 are fixedly mounted to the holder10. Each reference marker array 20 is operable to identify a landmark 16on the bone 14, and helps to track the position and the orientation ofthe landmark 16 through space. Each landmark 16 is a specific portion ofthe bone 14, in some instances a groove or a prominence on the bone 14,that serves as a guide to the location of other portions of the bone 14.Some non-limiting examples of bone landmarks 16 include the posteriorand superior iliac crest in the pelvis, the medial and lateral malleoliin the ankle, the femoral neck, and the femoral epicondyles.

Each reference marker array 20 is in fixed relationship with the holder10 such that the geometric relationship of each reference marker array20 with the holder 10, and with the other reference marker arrays 20, isknown. It will therefore be appreciated that since the position and theorientation of the bone 14 with respect to the holder 10 remainssubstantially constant, then the position and the orientation of thereference marker arrays 20 with respect to the bone 14 will also remainsubstantially constant.

In the depicted embodiment, each reference marker array 20 is spacedapart from the other reference marker array(s) 20. Each reference markerarray 20 is mounted to a portion of the boot 10A which is stationary,and thus does not undergo displacement relative to the structure of theboot 10A. Therefore, the reference marker arrays 20 do not undergodisplacement relative to one another, and thus their geometricrelationship remains substantially constant even as the boot 10Aundergoes movement.

In an alternate embodiment, one or more of the reference marker arrays20 are mounted to a portion of the boot 10A which undergoesdisplacement. Therefore, the reference marker arrays 20 undergodisplacement relative to one another in a known manner. Their geometricrelationship therefore varies in a known fashion with the displacementof the boot 10A, and thus the relative movement between the referencemarker arrays 20 can be compensated for.

Each reference marker array 20 has multiple reference markers 21. Theterm “reference marker” is intended to mean an active or passive marker,such as an emitter or a reflector. Each reference marker 21 is thereforean active or passive trackable object, and can operate using optical,RF, ultrasound, or electromagnetic signals. In FIG. 1, each referencemarker 21 is fixedly mounted to the holder 10. Therefore, the referencemarkers 21 do not undergo displacement relative to the holder 10 and toone another, and thus their geometric relationship remains substantiallyconstant even as the boot 10A undergoes movement. It will therefore beappreciated that since the position and the orientation of the bone 14with respect to the holder 10 remains substantially constant, then theposition and the orientation of the reference markers 21 with respect tothe bone 14 will also remain substantially constant.

In the illustrated embodiment of FIG. 1, each reference marker array 20includes three reference markers 21. The reference markers 21 aresubstantially flat and coplanar with the surface of the boot 10A. In analternate embodiment, one or more of the reference markers 21 protrudesfrom the surface of the boot 10A away therefrom. The reference markers21 of each array are located on a portion of the holder 10 whichapproximately overlies known bone landmarks 16. More particularly, inthe depicted embodiment, the reference markers 21 are located on theboot 10A to overlie approximate locations of the medial and lateralmalleoli 16A of the bones 14, i.e. the tibia and fibula. In embodimentswhere the holder 10 has a different form, the reference markers 21 arepositioned to overlie the approximate locations of other bone landmarks16.

Each reference marker array 20 is positioned on the holder 10 toidentify a landmark 16 of the bone 14 and thus assist in the tracking ofthe bone 14 through space. In the embodiment of FIG. 1, two or more bonelandmarks 16 are identified (i.e. the medial and lateral malleoli 16A).Typically, the greater the number of bone landmarks 16 that areidentified, the more accurately the bone 14 can be tracked. Differenttechniques for non-invasively identifying the bone landmark 16 arewithin the scope of the present disclosure, and some are now discussedin greater detail.

In the embodiment of FIG. 1, the reference markers 21 of each referencemarker array 20 are active. Stated differently, each reference marker 21is operable to emit an electromagnetic wave, to receive an echo of thewave off of a surface of the bone landmark 16, and to record a timemeasurement for the echo. This helps to calculate a distance of the bonelandmark 16 from each reference marker 21. In the illustrated embodimentwhere the holder 10 is in the form of a boot 10A and the referencemarkers 21 overlie approximate locations of the medial and lateralmalleoli 16A of the bone 14, the value of the calculated distance willbe indicative of the presence of the underlying medial and lateralmalleoli 16A. In an embodiment, the distance to the tips of the medialand lateral malleoli 16A is calculable, such that an ankle axis passingthrough the tips of the medial and lateral malleoli 16A can be located.The value of the calculated distance can be matched to known values ofthe distance between the outer-skin surface 11 and the medial andlateral malleoli 16A for the specific patient. Therefore, when the valueof the calculated distance (with appropriate compensation made for thethickness of the boot 10A) is substantially similar to the known valuesof the distance between the outer-skin surface 11 and the medial andlateral malleoli 16A, the calculated value will be indicative of thepresence of medial and lateral malleoli 16A landmarks, and thus thereference arrays 20 will have identified the landmark 16. The presenceof more than one reference marker 21 in each array 20 allowstriangulation to ensure the accuracy of the position of the landmark 16.In the illustrated embodiment, one or more of the reference markers 21includes an ultrasound device 22. The ultrasound device 22 is atransducer that emits an ultrasound wave and measures the time it takesfor the wave to echo off of a hard surface (such as the bone landmark16) and to return to the transducer face. Using the known speed of theultrasound wave, the time measurement is translated into a distancemeasurement between the reference marker 21 and the bone landmark 16located below the surface of the outer-skin surface 11. This distancemeasurement can then be compared to known distance measurements, aspreviously explained, to identify the bone landmark 16.

In an alternate embodiment where the reference markers 21 of eachreference marker array 20 are also active, the reference markers 21image the bone 14. Rather than using a measured or calculated distanceto identify the bone landmark 16, one or more of the reference markers21 in this embodiment operate the wave generation to produce sufficientecho to image the bone 14 in the vicinity of the reference marker 21.The localized image of the bone 14 is then compared to known images ofthe bones 14 to identify the bone landmark 16. In another alternateembodiment where the reference markers 21 of each reference marker array20 are active, one or more of the reference markers 21 produce a contourof the bone 14 in the vicinity of the reference marker 21. The localizedcontour of the bone 14 is then compared to contours in known images ofthe bone 14 to identify the bone landmark 16. The known images may beacquired using any appropriate imaging technique including radiography,magnetic resonance, etc. The known images may then be processed tocreate digital three-dimensional models of the bone or bones 14, for thesubsequent surface matching to be done to match the digitalthree-dimensional bone models to the data acquired by the referencemarkers 21 using ultrasounds or electromagnetic waves, for example. Inthe case of a digital three-dimensional model, a bone coordinate systemmay be virtually added to the model at pre-operative planning, forexample axes, planes, etc.

In alternate embodiments, the reference markers 21 of each referencemarker array 20 are passive and employ passive techniques to identifythe bone landmarks 16. In one such embodiment where the holder 10 is inthe form of a boot 10A, the passive reference markers 21 are positionedon the boot 10A in locations that are known to substantially overliebone landmarks 16, such as the medial and lateral malleoli 16A. Eachpassive reference marker 21 includes a reflector. The distance betweenthe reference markers 21 and the landmarks 16 can then be suitablyapproximated. As explained above, knowing this distance allows for thebone landmarks 16 to be tracked through space.

Referring to FIG. 2, there is also disclosed a system 30 for determininga position and an orientation of the bone 14 in space. The system 30 isa computer-assisted surgery (CAS) system. The system 30 includes one ormore wearable holders 10. In the illustrated embodiment, the wearableholder 10 is in the form of a belt 10B, but it may also be for examplethe boot 10A. The belt 10B is removably mountable about the outer-skinsurface 11 of an anatomical feature such as a leg having an underlyingfemur. The belt 10B is mounted about the leg to be positioned in fixedrelation with respect to the underlying femur. The system 30 alsoincludes the reference marker arrays 20 and the reference markers 21which are similar to those described above and will therefore not bediscussed in more detail. It will be noted that throughout the figures,like features are identified by like reference numerals.

The system 30 also includes a position sensing device 32. The positionsensing device 32 is used with the CAS system 30 to continuously trackthe position and/or orientation in space of the reference markers 21,and thus, of the bone 14, as explained in greater detail below. Theposition and orientation of the reference markers 21 may be tracked in areference coordinate system 34. According to an embodiment, thereference coordinate system 34 is defined by a fixed reference 36 thatis separate from the moveable holder 10. Many different configurationsof the position sensing device 32 which accomplish the above-describedfunctionality are possible and within the scope of present disclosure.For example, and as shown in the illustrated embodiment, the positionsensing device 32 is a distinct component which communicates with thereference markers 21 and with the fixed reference 36. In anotherembodiment, the position sensing device 32 is part of the fixedreference 36, as shown in FIG. 1 and described in greater detail below.

The fixed reference 36 has a known and a fixed position within thereference coordinate system 34 and is used to position and orient theholder 10, and thus the bone landmarks 16, in space within the referencecoordinate system 34. The fixed reference 36 is therefore any active orpassive device, with a known position in the reference coordinate system34. In the illustrated embodiment of FIG. 2, the fixed reference 36 isthe OR surgery table 36A which remains fixed in position within thereference coordinate system 34. A plurality of trackers 36B are fixedlymounted to the surgery table 36A. The trackers 36B are spaced apart fromone another, and are calibrated before the surgery to define thereference coordinate system 34. According to an embodiment, the plane ofthe table, represented by the trackers 36B, is assumed to be and enteredin the reference coordinate system 34 as being a frontal or sagittalplane of the patient lying in supine decubitus or lateral decubitus. Thetrackers 36B also communicate with the reference markers 21 on theholder 10 to track their position and orientation through space. It willtherefore be appreciated that when the holder 10 is fixedly mountedabout the anatomical feature, only the position and the orientation ofthe reference marker arrays 20 with respect to the fixed reference 36will change as the bone 14 moves through space. The holder 10 thusfunctions to lock the bone 14 and the reference markers 21 as a rigidbody allowing substantially no relative movement between thesecomponents.

In the embodiment of FIG. 1, the fixed reference 36 includes one or moreultrasound trackers 36C. The ultrasound trackers 36C are fixed inposition and spaced apart from each other to define the referencecoordinate system 34. The ultrasound trackers 36C form part of theposition sensing device 32. They are operable to emit ultrasound wavesand measure the time it takes for the wave to echo off of acorresponding reference marker 21 and to return to the ultrasoundtracker 36C. Using the known speed of the ultrasound wave, the timemeasurement is translated into a distance measurement between thecorresponding reference marker 21 and the ultrasound tracker 36C. Thisdistance measurement can then be used to determine the position andorientation coordinates of the reference marker 21 within the referencecoordinate system 34. Since the geometric relationship between thereference marker 21 and the landmark 16 is known, the distancemeasurement can also be used to determine the position and orientationcoordinates of the bone landmark 16 within the reference coordinatesystem 34. In alternate embodiments, the trackers 36B of the fixedreference 36 emit other electromagnetic signals (e.g. RF), or emitoptical signals.

In the illustrated embodiment of FIG. 1, the trackers 36B communicatewith the reference markers 21 on the holder 10 to generate position andorientation readings by triangulation. At least two reference trackers36B are fixed in position within the reference coordinate system 34.These at least two trackers 36B form two or more known points in thereference coordinate system 34, and a baseline length is determinedbetween them. A third point in the reference coordinate system 34, suchas the position of one of the reference markers 21, can then bedetermined with respect to the two points formed by the trackers 36B.Therefore, the position and/or orientation of the reference marker 21can be determined within the reference coordinate system 34. Thetrackers 36B therefore track the location of the holder 10 in thereference coordinate system 34, and thus the location of the bone 14fixed in position with respect to the holder 10, by triangulating theposition of one or more reference markers 21 fixed on the holder 10 withrespect to at least two reference trackers 36B in a known location inthe reference coordinate system 34. Although the reference markers 21are described as having the dual function of identifying landmarks, andbeing tracked, it is contemplated to provide other markers 21, such aspassive markers, that will be tracked by the position sensing device 32.

Referring again to FIG. 2, the CAS system 30 also has a processing unit40. The processing unit 40 is in communication with the position sensingdevice 32 to process the position and orientation readings of thereference markers 21. In some embodiments, the processing unit 40 is apart of the position sensing device 32. The processing unit 40 thereforeincludes any number of suitable components for performing the abovefunctionality, such as an internal central processing unit (CPU), amemory unit, and a storage unit. The processing unit 40 can be any of anumber of computing devices running a variety of applicable operatingsystems. The processing unit 40 may also include a display device, suchas a monitor. The processing unit 40 may also include one or more inputdevices such as keyboards, pointing devices, and the like. Theprocessing unit 40 runs various modules, in the form of algorithms,code, non-transient executable instructions, etc., in order to operatethe CAS system 30 in the manner described herein.

Referring to the embodiment of the processing unit 40 shown in FIG. 3,the processing unit 40 includes, or communicates with, one or moreinterfaces D, for the information to be provided to the operator. Theinterfaces D may be monitors and/or screens including wireless portabledevices (e.g., phones, tablets), audio guidance, LED displays, amongmany other possibilities. For example, the interface D comprises agraphic user interface (GUI) operated by the CAS system 30. Theinterface D may also display images outputted by the processing unit 40,for instance to track the bone 14 through space.

The processing unit 40 has a landmark identification module 41. Thelandmark identification module 41 is configured to output informationindicative of the bone landmarks 16 to be tracked. In operation, thelandmark identification module 41 receives data from the referencemarkers 21 of the reference arrays 20 and processes the data to identifythe bone landmarks 16. The landmark identification module 41 is operableto process the data using any one of the different techniques fornon-invasively identifying the bone landmark 16 described above. In anembodiment, the landmark identification module 41 commands the referencemarkers 21 or components thereof (e.g. ultrasound devices 22) to emitwaves (electromagnetic, ultrasound) and then measures the time it takesfor the wave to echo off of the bone landmark 16 and to return to thereference marker 21. Using the known speed of the wave, the landmarkidentification module 41 translates the time measurement into a distancemeasurement between the reference marker 21 and the bone landmark 16located below the surface of the outer-skin surface 11. The landmarkidentification module 41 then compares this distance measurement toknown distance measurements, for instance for confirmation, and outputsthe identified bone landmark 16. The landmark identification module 41may use triangulation knowing the geometrical arrangement of thereference markers 21 in the array 20.

The landmark identification module 41 has a virtual bone module 41A. Thevirtual bone module 41A provides the landmark identification module 41with an image of the bone 14 and/or the bone landmark 16. The virtualbone module 41A in an embodiment therefore receives image data of thebone 14 and/or landmark 16 from the reference markers 21. In analternate embodiment, the virtual bone module 41A includes a database ofimages of bones 14 any one of which can be provided to the landmarkidentification module 41 to identify a bone landmark 16.

The processing unit 40 also has a holder tracking module 42. The holdertracking module 42 is configured to output position and orientationreadings of the reference markers 21 or like markers on the holder 10 inthe reference coordinate system 34. The position and orientationreadings help to determine the position and orientation of the bonelandmark 16 in the reference coordinate system 34, and thus help totrack the bone 14 in the space. The holder tracking module 42 receivesactive feedback from the trackers 36B or 36C to determine the positionof the reference markers 21 within the reference coordinate system 34.The holder tracking module 42 is operable to process data using any oneof the different techniques for tracking the reference markers 21 orlike holder markers described above. For example, the holder trackingmodule 42 can triangulate the position and orientation readings of thereference markers 21 with the two or more reference trackers 36B fixedin position within the reference coordinate system 34. Moreparticularly, the holder tracking module 42 calculates the baselinelength between the trackers 36B in the reference coordinate system 34.The holder tracking module 42 then determines the position andorientation of a third point in the reference coordinate system 34 (i.e.one of the reference markers 21) using triangulation with respect to thetwo points formed by the trackers 36B.

The processing unit 40 also has landmark tracking module 43. Thelandmark tracking module 43 in operation receives the position andorientation readings of the reference markers 21 from the holdertracking module 42. The landmark tracking module 43 in operation alsoreceives the landmark 16 identification information from the landmarkidentification module 41. In an alternate embodiment, the landmarktracking module 43 in operation receives an image of the bone 14 and/orthe bone landmark 16 from the virtual bone module 41A. With thisinformation, the landmark tracking module 43 is able to generate data onthe position and orientation of the bone landmarks 16 in order to trackthe position and/or orientation of the bone 14 within the referencecoordinate system 34. The position and orientation readings of thereference markers 21 allow the landmark tracking module 43 to determinethe position and orientation of the holder 10 in the referencecoordinate system 34 because the geometric relationship between thereference markers 21 and the holder 10 is known. Since the position andorientation of the holder 10 within the reference coordinate system 34is known, the landmark tracking module 43 is operable to determine theposition and/or orientation of the bone landmarks 16 in the referencecoordinate system 34 because the geometric relationship between the bonelandmarks 16 and the holder 10 is known. Therefore, the landmarktracking module 43 is operable to track the bone 14 and its landmarks 16through space within the reference coordinate system 34. The landmarktracking module 43 outputs position and orientation readings of the bone14 to the interface D.

It will therefore be appreciated that the CAS system 30 disclosed hereinallows the position and/or orientation of a bone 14 to be tracked in areference coordinate system 34 by simply tracking the movement of aholder 10 mounted securely and non-invasively about the outer-skinsurface 11 of the anatomical feature containing the bone 14. Theposition and orientation of the holder 10 is therefore actively trackedin the CAS system 30, and from the position and orientation readings ofthe holder 10, the CAS system 30 indirectly determines the positionand/or orientation of the bone 14 underlying the holder 10 and in fixedgeometric relationship therewith. In at least some of the embodimentsdescribed above, the CAS system 30 determines the position and/ororientation of the bone 14 without having to continuously image the bone14 itself, which can reduce system processing times. Indeed, the CASsystem 30 allows for bone tracking without having to continuouslyidentify the bone landmark 16. The CAS system 30 and holder 10 can beused to identify the bone landmarks 16 only once, and because of thefixed relationship between the bone landmarks 16 and the holder 10, theCAS system 30 can operate to track only the holder 10 in order todetermine the position and orientation of the bone 14 in space. Ifdesired, the operator can update the identification of the bone landmark16.

The holder 10 also helps to overcome problems encountered with relativemovement between outer-skin surface 11 and bone 14. The holder 10 isfixedly mounted to the outer-skin surface 11, and thus reduces relativemovement between the outer-skin surface and the holder 10. Thiscontrasts with some conventional techniques for non-invasively trackingbones which rely on garments, socks, and the like. These relativelyloose-fitting coverings can move with the outer-skin surface and aretherefore not in fixed relation with the bone or anatomical feature.

FIG. 4 illustrates another embodiment of the system 130 for determininga position and an orientation of the bone 114 in the referencecoordinate system 134. The system 130 also includes a wearable holder110, reference marker arrays 120, and reference markers 121 which aresimilar to those described above and will therefore not be discussed inmore detail. It will be noted that in FIG. 4 like features areidentified by like reference numerals.

In the depicted embodiment, the CAS system 130 includes two wearableholders 110 or probes, each fixedly mounted about one of the bones 114shown (i.e. the femur 114A and the tibia 114B). A first probe 110A inthe form of a belt is mountable about the outer-skin surface 11 of anupper portion of the leg having the underlying femur 114A. A secondprobe 110B, also in the form of a belt, is mountable about theouter-skin surface 11 of a lower portion of the leg having theunderlying tibia 114B. The probes 110A, 110B therefore allow tracking ofthe bones of the entire leg of the patient. More or fewer holders 110can be used in the CAS system 130, and different bones 114 can betracked in the CAS system 130.

In the depicted embodiment, each holder 110 has a plurality of referencemarkers 121 fixedly mounted to the holder 110. The position and theorientation of the reference markers 121 is trackable in the referencecoordinate system 134, as explained above, so as to determine theposition and the orientation of the holders 110 in the referencecoordinate system 134. By tracking the holders 110 or probes, theposition and orientation of the bone landmarks 116 may also be tracked.

The CAS system 130 includes a bone imaging device 131. The bone imagingdevice 131 operates to obtain a pre-operative image of the bone 114 inits entirety. The position and the orientation of the bone 114 when thepre-operative image is obtained is known in the reference coordinatesystem 134. The bone imaging device 131 can take many differentconfigurations to accomplish the above-described functionality, some ofwhich are now described in greater detail.

In a particular embodiment, the bone imaging device 131 is operable toconstruct the pre-operative image of the bone 114 from at least twoimages of the bone 114 taken in non-coplanar planes. For example, if thebone imaging device 131 includes an X-ray imager, it operates togenerate views of the bone 114 in two orthogonal planes. The missinginformation of the bone 114 can then be constructed from the views inthe orthogonal planes. Other CT (computed tomography) imaging techniquesare also possible. Magnetic resonance imaging (MRI) can also be used.

In an alternate embodiment, the bone imaging device 131 includes alibrary of pre-operative bone images. A suitably correspondingpre-operative bone image can be retrieved from the library of imagesbased on the particular characteristics of the patient (e.g. age, sex,race, height, weight, etc.). In yet another alternate embodiment, thebone imaging device 131 operates to obtain pre-operative images of thebone landmarks 116, and then constructs the bone 114 based on the imagedbone landmarks 116. Irrespective of the technique used to obtain thepre-operative image of the bone 114, the pre-operative image provides a“map” against which comparisons can be made. Stated differently, thepre-operative image of the bone 114 provides the position andorientation of the bone 114 and all its surfaces within the referencecoordinate system 134. Comparing images taken during surgery to thepre-operative image helps to track the position and/or the orientationof the bone 114 in the reference coordinate system 134 during surgicalprocedures.

The CAS system 130 also includes one or more holder imaging devices 133.Each holder imaging device 133 is fixedly mounted to the holder 130 andoperates to obtain an image of a landmark 116 of the bone 114 atdifferent time intervals. The image of the landmark 116 can then becompared to the pre-operative image of the bone 114 to determine theposition and/or orientation of the bone 114 within the referencecoordinate system 134 at a given time interval.

In the depicted embodiment, the images produced by each holder imagingdevice 133 are in the form of a curve or contour of a portion the bone114. The curve image is obtained by sweeping the bone 114 with theholder imaging devices 133. The holder imaging devices 133 therefore mapin detail a portion of the bone 114. In an alternate embodiment, theimage produced by each holder imaging device 133 is a three-dimensionalimage or reconstruction of the portion of the bone 114. It will beappreciated that the more accurate the image produced by each holderimaging device 133, the greater likelihood that it will be matched withthe pre-operative image. In the depicted embodiment, each holder imagingdevice 133 is an ultrasound imaging device. In the depicted embodiment,each holder 110 has six holder imaging devices 133. More or fewer holderimaging devices 133 are within the scope of the present disclosure. Theimages of the landmark 116 and the pre-operative image of the bone 114can be illustrated on an interactive or a non-interactive displaydevice.

The processing unit 140 of the CAS system 130 is operable to receive theimage of the landmark 116. The processing unit 140, or a module thereof,compares the image of the landmark 116 at a given time interval to thesame landmark in the pre-operative image. The position and/or theorientation of any portion of the bone 114 in the reference coordinatesystem 134 at the time interval can therefore be determined by matchingthe image of the landmark 116 to the same landmark in the pre-operativeimage. Stated differently, the processing unit 140 operates to“register” features of the imaged bone 114 with the pre-operative imageof the same bone 114. In recognizing the pattern of the bone landmarks116 by matching the images, any movement (e.g. translation, rotation,etc.) of the bone 114 underlying the holder 110 with respect to thepre-operative image can be tracked in the reference coordinate system134. Any shifts between the image of the landmark 116 and the landmarkof the pre-operative image help to determine the position and/ororientation of any portion of the bone 114 relative to the pre-operativebone 114, and thus the position and/or orientation of any portion of thebone 114 in the reference coordinate system 134. The CAS system 130therefore helps to build links with features of the bone 114 which thesurgeon needs to track during surgical procedures, without having toactively track those features. Instead, the CAS system 130 can tracklandmarks 116 that are unimportant from a surgical perspective, and fromthis information, provide the position and orientation of features ofthe bone 114 that are of interest to the surgeon.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the invention disclosed.Still other modifications which fall within the scope of the presentinvention will be apparent to those skilled in the art, in light of areview of this disclosure, and such modifications are intended to fallwithin the appended claims.

1. A system for determining a position and an orientation of a bone ofan anatomical feature, the bone underlying an outer-skin surface of theanatomical feature, the system comprising: a wearable holder configuredto be mounted about the outer-skin surface of the anatomical feature,the anatomical feature and the bone being positioned in fixed relationwith respect to the wearable holder when the wearable holder is mountedabout the anatomical feature; at least two reference marker arraysfixedly mounted to the wearable holder, each said reference marker arraybeing positioned on the wearable holder to identify a landmark of thebone within the wearable holder when the wearable holder is mountedabout the anatomical feature, each said reference marker array includinga plurality of reference markers; a fixed reference defining a referencecoordinate system; a position sensing device operable to registerposition and orientation readings of the reference markers in thereference coordinate system; and a processing unit operable to receivethe position and orientation readings and to measure time intervals, theprocessing unit being operable to determine the position and theorientation of the holder in the reference coordinate system for a giventime interval.
 2. The system as defined in claim 1, wherein the wearableholder includes a boot being removably mountable about a foot and alower leg.
 3. The system as defined in claim 2, wherein the arrays arepositioned on the boot to overlie the landmarks of the foot and lowerleg, the landmarks including medial and lateral malleoli of the anklewhen the boot is mounted about the foot and the lower leg.
 4. The systemas defined in claim 2, wherein the reference marker arrays are fixedlymounted to an immobile portion of the boot.
 5. The system as defined inclaim 1, wherein the reference markers include active reference markers,each reference marker being operable to emit an electromagnetic wave,receive an echo of the wave off of a surface of the landmark, and recorda time measurement for the echo.
 6. The system as defined in claim 1,wherein the fixed reference includes a plurality of trackers fixedlymounted to a surgery table, each tracker being spaced apart from oneanother to define the reference coordinate system.
 7. The system asdefined in claim 1, wherein at least one of the reference markersincludes an ultrasound device.
 8. The system as defined in claim 1,wherein the fixed reference includes at least one ultrasound tracker. 9.The system as defined in claim 1, wherein the holder includes a beltbeing removably mountable about a leg.
 10. A method for determining aposition and an orientation of a bone of an anatomical feature, the boneunderlying an outer-skin surface of the anatomical feature, the methodcomprising: removably attaching in a non-invasive manner a holder aboutan anatomical feature having the bone to fix a position of theanatomical feature and the bone with respect to the holder; identifyingat least two landmarks of the bone, the landmarks being fixed inposition with respect to the holder; registering position andorientation readings of reference markers fixedly mounted to the holderin a reference coordinate system; and determining the position and theorientation of the holder using the position and orientation readings ofthe reference markers to thereby determine the position and orientationof the landmarks of the bone in the reference coordinate system.
 11. Themethod as defined in claim 10, wherein identifying the at least twolandmarks includes identifying the at least two landmarks withoutimagery of the bone.
 12. The method as defined in claim 10, whereinidentifying the at least two landmarks includes measuring a distancebetween each reference marker and the corresponding landmark on thebone.
 13. The method as defined in claim 10, further comprising trackingthe landmarks of the bone by repeating said registering and saiddetermining at different time intervals.
 14. The method as defined inclaim 10, wherein registering the position and orientation readingsincludes triangulating the position and orientation readings of thereference markers with at least two reference trackers fixed in positionwithin the reference coordinate system.
 15. The method as defined inclaim 10, wherein identifying the at least two landmarks includespre-operatively imaging the at least two landmarks.
 16. A wearableholder that is trackable by a computer-assisted surgery system, thewearable holder comprising: a body removably mountable about anouter-skin surface of an anatomical feature having an bone, theanatomical feature and the bone being positioned in fixed relation withrespect to the body when the body is mounted to the anatomical feature;and at least two reference marker arrays fixedly mounted to the body,each reference marker array being positioned on the body and operable toidentify a landmark of the bone when the body is mounted about theanatomical feature, each of the reference marker arrays having aplurality of reference markers, a position and an orientation of thereference markers being trackable in a reference coordinate system todetermine the position and the orientation of the body in the referencecoordinate system.
 17. A system for determining a position and anorientation of a bone of an anatomical feature, the bone underlying anouter-skin surface of the anatomical feature, the system comprising: awearable holder removably mountable about the outer-skin surface of theanatomical feature, the anatomical feature and the bone being positionedin fixed relation with respect to the holder when the wearable holder ismounted about the anatomical feature; at least one holder imaging devicefixedly mounted to the wearable holder to obtain an image of a landmarkof the bone at a given time interval; a bone imaging device operable toobtain a pre-operative image of the bone in its entirety, the positionand the orientation of the bone in the pre-operative image being knownin a reference coordinate system; and a processing unit operable tocompare the image of the landmark at said time interval to the samelandmark in the pre-operative image, the position and the orientation ofany portion of the bone in the reference coordinate system at said timeinterval being determined by matching the image of the landmark to thesame landmark in the pre-operative image.
 18. The system as defined inclaim 17, wherein the bone imaging device is operable to construct thepre-operative image of the bone from at least two images taken innon-coplanar planes.
 19. The system as defined in claim 17, wherein theimage of the landmark obtainable by the at least one holder imagingdevice is a contour of the bone underlying the wearable holder.
 20. Thesystem as defined in claim 17, further comprising a plurality ofreference markers fixedly mounted to the holder, the position and theorientation of the reference markers being trackable in the referencecoordinate system to determine the position and the orientation of theholder in the reference coordinate system.
 21. The system as defined inclaim 17, wherein the wearable holder includes a first probe mountableabout the outer-skin surface of an upper portion of the leg having anunderlying femur, and a second probe mountable about the outer-skinsurface of a lower portion of the leg having the underlying tibia. 22.The system as defined in claim 17, wherein the at least one holderimaging device includes an ultrasound imaging device.
 23. A method fordetermining a position and an orientation of a bone of an anatomicalfeature, the method comprising: obtaining a pre-operative image of thebone in its entirety, the position and the orientation of the bone inthe pre-operative image being known in a reference coordinate system;removably attaching in a non-invasive manner a holder to the anatomicalfeature to fix a position of the anatomical feature and the bone withrespect to the holder; obtaining an image of a landmark of the bone at agiven time interval with the holder, the landmark being fixed inposition with respect to the holder; and determining the position andthe orientation of any portion of the bone in the reference coordinatesystem at said time interval by matching the image of the landmark tothe same landmark in the pre-operative image.
 24. The method as definedin claim 23, wherein obtaining the pre-operative image includesconstructing the pre-operative image from at least two images taken innon-coplanar planes.
 25. The method as defined in claim 23, whereinobtaining an image of the landmark includes obtaining a contour of thebone underlying the holder.
 26. The method as defined in claim 23,further comprising registering position and orientation readings ofreference markers fixedly mounted to the holder in the referencecoordinate system, and determining the position and the orientation ofthe holder using the position and orientation readings of the referencemarkers to thereby determine the position and orientation of thelandmarks of the bone in the reference coordinate system.